Comparisons of pre-lethal neurochemical alterations to neurologic outcome and neuropathology following cardiac arrest (CA) and resuscitation using hyperoxic and normoxic ventilation strongly implicate oxidative modification to mitochondrial proteins and associated bioenergetic dysfunction in the etiology of delayed, selective neural cell death. These findings also question the indiscriminate use of 100% ventilatory O2 (FiO2) with patients following CA and suggest that modification of existing resuscitation guidelines may significantly improve neurologic outcome. Our primary goal is to reduce neurologic morbidity and mortality following CA through by minimizing oxidative stress and maximizing cerebral energy metabolism. Our specific aims are to test the following hypotheses focusing on mitochondrial mechanisms of oxidative brain injury in young and aged animals, and on optimizing neurologic outcome using oximetry-based adjustments to FiO2 that are practical for use in out-of-hospital CA. 1. Oxidative brain injury and neurologic impairment following cerebral ischemia are minimized by maintaining postischemic hemoglobin O2 saturation at 94 - 98%. 2. Post-resuscitative cerebral hyper-oxygenation worsens neurologic and histopathologic outcome as a consequence of impaired cerebral energy metabolism, delayed neuronal Ca2* dysregulation, and exacerbated expression and subcellular redistribution of pro-apoptotic proteins. 3. Neuronal survival following in vitro hypoxia and re-oxygenation is optimized using moderate post-hypoxic oxygenation, due to reduced oxidative stress-mediated mitochondrial dysfunction. 4. Aged animals are sensitive to exacerbation of oxidative stress, cell death, and neurologic impairment by post- resuscitative hyper-oxygenation. Methods of approach include the use of mature and aged animals in models of global cerebral ischemia, models of cell death using primary neuronal cultures, measurements of mitochondrial Ca2+ transport, membrane potential, and production of reactive 02 species with brain mitochondria and neurons, immunohistochemical and immunoblot analysis of changes in nitrotyrosine and the levels and intracellular distribution of metabolic and apoptotic proteins, 13C NMR spectroscopic analysis of altered metabolism in the post-ischemic brain and in primary cultures of neurons exposed to stress, and short- and long-term tests of neurologic impairment.